The present invention relates to an image recording apparatus. More particularly, the present invention relates to an image transfer unit which comprises a transfer belt disposed along a plurality of photoconductors and transfers toner images from each of the photoconductors to the transfer belt or a paper placed on and conveyed by the belt.
An electrophotographic multi-color image recording apparatus comprises an image transfer unit which generally utilizes a technique in which toner images of different colors separated from an original color are superposed one above the other to form a multi-color image corresponding to the original. A conventional image transfer unit comprises for example four photoconductor drums on which color data of an image to be recorded with respect to yellow, magenta, cyan and black are written to form a latent image of a respective different color thereon and developing devices each arranged in connection with each of the four photoconductor drums, respectively. The original color is separated to three colors, i.e., blue, green and red each of which is detected by a respective optical sensor. An image processor calculates color data of yellow, magenta, cyanogen and black on the basis of the luminous intensity of each of the separated three colors. The electrostatic latent images formed on the photoconductor drums are developed by a toner of a corresponding color contained in the respentive corresponding developing device. A record paper is fed onto a transfer belt from a paper container through a resist roller and conveyed by the transfer belt which circulates along the photoconductor drums and comes in contact therewith one after another. The record paper is electrostatically sticked and secured to the conveyor belt surface. The visible toner images formed on the photoconductor drums are transferred to the record paper by a function of a transfer charger disposed behind the transfer belt at the position of each photoconductor drum in such a manner that the different color toner images are superposed one above the other to form an image of multi-colors on the record paper. After that, the multi-color image is fixed on the paper by a fixing roller.
Such a transfer unit comprising the transfer belt which is generally made from a dielectric material requires a back up means for supporting the transfer belt from behind thereof so that the belt or the paper carried by the belt reliably comes in contact with the photoconductor surface. Such a back up means generally comprises rollers which are disposed behind and along the transfer belt and driven to rotate at the same speed as the transfer belt or move along with the belt so as to decrease the load upon the transfer belt. Such kind of back up rollers are usually made from a metal so that the rollers are easily processed to accurately form a cylindrical shape and grounded to the earth.
The transfer belts are electrostatically charged by the transfer charger at the time of the transferring operation. Since the metallic back up rollers are in contact with the transfer belt, the image to be transferred from the photoconductor to the transfer belt or the paper placed on the belt is apt to be disturbed and deformed, as described in Japanese Examined Patent Publication Nos. 55-33072 and 56-97357. The reason for this is that when the transfer belt suddenly comes in contact with the back up roller, a disturbance electric field is generated between the belt and the back up roller so that the transferring electric field applied to the toner is disturbed whereby the toner is scattered especially in the horizontal direction. Such a disturbance electric field is also generated at the time when the belt is moved away from the back up roller. Also, according as the transfer belt moves away from the back up roller, the electric voltage between the transfer belt and the back up roller rises which causes to generate a separation discharge therebetween so that the electric charge charged behind the transfer belt is eliminated which extinguishes the electrostatic force to hold the toner which therefore is scattered.